(a) Field of the Invention
This invention relates to a composition for filling communications cables, a communication cable containing the composition, a method of making the composition and a method of making a communication cable.
(b) Description of Prior Art
Communication cables generally comprise a plurality of conductors which may or may not be individually or collectively insulated and which may or may not include a core, enclosed within a waterproof sheath; the interstices between the conductors and between the conductors and the sheath being filled from end to end of the cable length with a water-impervious medium. The water-impervious medium should not drain under the influence of gravity or such hydrostatic pressure as may arise in the event of damage to the cable sheath, as this would leave an incompletely filled cable along which moisture might travel and the medium should further permit relative sliding movement of the conductors over one another during such bending of the cables as occurs during manufacture and installation of the cable.
A preferred water-impervious medium is petroleum jelly, however, petroleum jelly has a tendency to seep from the cable ends or from a flaw developed in the cable, at elevated temperatures to which the cable might be subjected in installation or in use particularly in warm climates.
Various attempts have been made to overcome this disadvantage of petroleum jelly. Compositions comprising petroleum jelly with the addition of micro-crystalline waxes and low molecular weight polyethylene resin have partially solved the problem, by rendering the petroleum jelly firm at room temperatures of the order of 20.degree. C.; and reducing the mobility of the petroleum jelly at higher temperatures of the order of 70.degree. C. sufficiently to prevent any substantial seepage of the petroleum jelly from the cable.
These compositions, however, are not entirely successful because the petroleum jelly and the wax and resin are not entirely compatible and there is a tendency for them to separate, particularly during the filling process which may involve sustained high temperatures accompanied by mechanical shearing.
Further in cables in which the conductors are electrically insulated with a cellular insulation, such as cellular polyethylene, there is a tendency for the filler composition to migrate into the cellular insulation and the petroleum jelly may itself swell the carcass of the cells of the cellular insulation. In the case of cellular insulating materials, for example, cellular polyethylene, this migration may continue until the cells of the insulating material are filled. The filling of the cells affects the electrical properties of the cable particularly the capacitance.
It is an object of the present invention to provide a composition suitable for filling telecommunication cables which will overcome the aforementioned problems.
It is a further object to provide communication cables, particularly telecommunication cables contain the composition.
It is a further object of the invention to provide a method of making a composition which will overcome the aforementioned problems.
It is a further object of the invention to provide a method of making a communication cable, particularly telecommunication cables containing the composition.